The present invention relates to the field of microwave thermal ablation therapy of tissue. In particular, the present invention relates to a catheter for microwave thermal ablation therapy of cardiac tissue to treat cardiac arrhythmias.
Cardiac arrhythmias are one type of irregular beating of the heart which can result from damage caused to a portion of cardiac tissue. Specifically, cardiac arrhythmias result when damaged cardiac tissue (e.g., along a ventricle or atrial wall) prevents the proper conduction of an electrical and rhythmic impulse through the ventricle wall or atrial wall in the region of damaged tissue. This disruption of the electrical and rhythmical impulse inhibits the chambers of the heart from contracting and expanding with the proper timing and the proper force.
Cardiac arrhythmias have traditionally been treated with medication and/or through surgery. However, medications are limited in their application since they are successful in treating only certain types of cardiac arrhythmias. On the other hand, while a portion of cardiac tissue which is damaged can be surgically removed, surgery carries much more risk than medicinal treatment of cardiac arrhythmias.
Alternative methods of treating cardiac arrhythmias include applying heat to the damaged cardiac tissue by inserting a catheter into a chamber of the heart, e.g., ventricle, and using the catheter to apply heat locally to ablate the damaged portion of cardiac tissue to neutralize its effect on the electrical and rhythmical impulse. For example, Fram PCT International Publication WO 94/07446, published Apr. 14, 1994, discloses a catheter and method for ablating electrically conductive pathways of a heart. This catheter includes a balloon mounted on a catheter shaft and a heating device located within the balloon to heat the fluid inside the balloon. The heated fluid heats the cardiac tissue by thermal conduction from the balloon fluid to the tissue through a wall of the balloon. This technique carries several disadvantages. First, since this method uses thermal conduction, the surface of the cardiac tissue immediately adjacent the outer surface of the balloon is necessarily heated yet cardiac tissue deep below the surface, where lesions are frequently located, remains unheated. Second, since the catheter necessarily heats the surface of the cardiac tissue, any healthy tissue on the surface of a wall of the cardiac chamber is unnecessarily damaged. Finally, this method of applying heat to cardiac tissue unnecessarily heats blood in the cardiac chamber (e.g., ventricle) surrounding a side of the catheter opposite a wall of the chamber. This technique can coagulate small portions of blood that pass through the particular chamber of the heart adjacent the catheter, thereby producing clotting and emboli in the bloodstream.
Other catheters ablate cardiac tissue using radio frequency or microwave energy that is transmitted from a distal end of the catheter through cardiac tissue to the site to be treated. Langberg U.S. Pat. No. 4,945,912 discloses a catheter including a microwave antenna for directing microwave energy to ablate cardiac tissue than is possible with conventional thermal conduction methods such as those disclosed in the Fram PCT Publication. However, these catheters also unnecessarily necrose healthy tissue immediately adjacent an outer surface of the catheter.
In an attempt to limit heating of tissue immediately adjacent a heat generating portion of a catheter, some cardiac ablation catheters include cooling systems incorporated into the catheter adjacent the heat generating portion of the catheter. Nardella, U.S. Pat. No. 5,334,193, discloses an ablation catheter which includes at its distal end, an electrode for directing radio frequency energy (RF) to necrose damaged cardiac tissue and a cooling lumen centrally aligned throughout the catheter for delivering cooling fluid adjacent the electrode for limiting heat transferred by the electrodes to adjacent tissues.
Other methods of protecting healthy tissue from the heat generated by an electrode of a microwave ablation catheter include attempts to selectively block the propagation of the microwave field generated by the electrode. Stern et al., U.S. Pat. No. 5,314,466, discloses a microwave ablation catheter including a structure for blocking the propagation of a microwave field in a desired direction to prevent undesired heating of blood within the cardiac chamber which surrounds the heat producing end of the catheter. This catheter also has a mechanism for pivoting a distal portion of its catheter relative to a remaining proximal portion of the catheter to orient the microwave field in a desired orientation relative to the tissue site to be treated.
Previous cardiac ablation catheters have attempted to limit the heat applied to tissues and/or blood immediately surrounding the heat producing end of the catheter. However, thus far, previous catheters have failed to: (1) deliver adequate microwave energy in cardiac tissue at depths which actually necrose the damaged tissue; (2) adequately cool healthy tissue immediately adjacent the microwave energy producing portion of the catheter; and (3) adequately protect a cardiac environment, such as blood, immediately surrounding an outer surface of the microwave energy producing portion of the catheter.